1. Field of the Invention
This invention relates to the field of precision stages, and in particular to stages compatible with high voltage and high vacuum, for use in multi-column charged particle lithography, test and inspection systems.
2. Description of the Related Art
Precision stages find many applications with steadily increasing precision requirements. In semiconductor capital equipment, for example, precision stages are required for carrying wafers during lithography process steps. As the length scales of microcircuit features become smaller, modern lithography moves toward the use of higher resolution techniques, for example phase shift masks, extreme ultraviolet (EUV) systems, and high throughput charged particle beam lithography. With the overall trend toward smaller features, positioning requirements of the wafer (and a stage platform upon which it rests) relative to lithography optics have become increasingly stringent.
Due to the comparative magnitudes of charged particle wavelengths and sub-micron features, charged particle (especially electron) beam lithography is an important technique for realizing sub-micron feature sizes. However, in charged particle beam lithography systems, vacuum and high voltage compatibility is an inherent design complication.
In order to accelerate a charged particle beam, for example an electron beam, particle beam lithography systems typically have electric potential differences (up to 100 kV) between different components. In many designs, a workpiece or wafer is held at a potential close to electrical ground and the electron source is at comparatively higher voltage. However, in some systems, the electron source is operated at a potential close to electrical ground, while the workpiece is held at comparatively higher voltage. Such an electron source is described in U.S. Pat. No. 5,637,951.
A vacuum is also required within the lithography device to allow the propagation of an electron beam. A high vacuum, with pressure of no more than 10xe2x88x926 Torr, is typically required. Thus, in addition to providing the required precision, a stage for electron beam lithography must be able to sustain high voltage differences between components and be suitable for operation under high vacuum conditions.
In regard to high voltage operability, many prior art precision stages comprise several closely coupled kinematic platforms. An exemplary prior art device of this type is shown in FIG. 1. However, in a system where up to 100 kV potential difference must be held off with no arcing or other undesirable effects, close mechanical coupling of non-insulating components, such as illustrated in FIG. 1, would be inappropriate.
In regard to vacuum compatibility, many prior art stages are inappropriate for use in a high vacuum due to bearing designs that are prone to out-gassing, generating particulates, or otherwise contaminating the vacuum system. In comparison to other bearing types, flexural bearings are very well-suited for use in vacuum, and allow smooth, precise, predictable motion of the stage. Such properties are discussed in detail by Alexander Slocum, xe2x80x9cPrecision Machine Designxe2x80x9d, Prentice Hall, N.J., 1992. However, most prior art flexural bearings are potentially unstable when used as thrust bearings since the flexural joint is prone to distortion or buckling under compressive loads. Thus, in precision stage applications requiring vacuum compatibility, there is a need for multiple degrees of freedom of movement flexural thrust joints.
Further, for positioning systems requiring precision movement and short times to reach a steady-state position (settling times) after each movement, attention must be paid to perturbations that are insignificant for less precise systems. In order to optimize the mechanical stability of the stage under external impulses, vibrations or other mechanical perturbations, most prior art high precision lithography stages utilize massive, high inertia elements. However, while a high inertia system tends to be mechanically stable, the reaction forces associated with moving a massive stage can degrade the accuracy and precision of the relative positioning of the lithography optics and the work platform, unless a long settling time is accepted. A combination of careful isolation of the optics and addition of considerable mass to the optics mounts is required to overcome this problem. The resulting weight and volume of such devices is undesirable. It would be beneficial, then, to have a comparatively low-mass high precision stage that avoids these problems. Furthermore, such a low-mass stage would be well-suited for commercial lithography tools with their high throughput requirements, where frequent large accelerations and decelerations are required for rapid processing of the workpiece.
Therefore, especially for use in charged particle lithography systems, there is a need for stages capable of maintaining high precision alignment with a reference (for example the position of the lithography optics) while being vacuum compatible and capable of sustaining large voltage differences between elements. Such platforms should be continuously and smoothly movable over six degrees of freedom of movement. Such platforms should also be capable of holding workpieces with characteristic lengths of hundreds of millimeters, e.g. 300 mm silicon wafers.
This invention includes a precision stage incorporating a novel, legged design. The stage may be used in applications requiring high voltage and vacuum, such as charged particle lithography. According to aspects of the invention, the stage comprises: a base; a frame attached to the base; a platform with a bottom platform surface; at least three adjustable limbs coupled to the base and the bottom platform surface, each limb comprising a raising member attached to the base, a first attachment member attached to the raising member, a leg, a bottom end of the leg attached to the first attachment member, a second attachment member attached to a top end of the leg and the second attachment member attached to the bottom platform surface; and platform movement members coupled to the platform and the frame. The attachment members can have two or three degrees of freedom of movement; they can be flexure joints; they can be flexural thrust joints. Furthermore, the first and second attachment members can be distinct from each other, by the degrees of freedom or the type of joint. The platform movement members can be actuators, and at least three actuators are provided for the platform. Further, they can be electromagnetic platform actuators, comprising: a first coil attached to the platform; a first magnet assembly attached to the frame and electromagnetically coupled to the first coil; and, a current control system electrically coupled to the first coil. Furthermore, a cooling system and cooling channels can be thermally coupled to the component of the electromagnetic actuator attached to the platform, allowing for platform temperature control to within +/xe2x88x920.1 degrees C. The raising members can be a combination of elements, arranged in parallel, chosen from: support elements, which provide a force to oppose the weight of the platform; raising actuators, which allow adjustment of the elevation of the platform relative to the base; and damping elements. The support elements can be constant force supports, and the constant force supports can be buckled columns. The raising actuators can be electromagnetic raising actuators, comprising: a second coil attached to the base; a second magnet assembly attached to one of the first attachment members and electromagnetically coupled to the second coil; and, a current controller coupled to the second coil.
In preferred embodiments: the platform has six degrees of freedom of movement; the legs are substantially parallel to each other; the platform is capable of accommodating wafers with a diameter of at least 100 mm; the platform and/or legs are made of substantially electrically insulating material, and/or the frame is a vacuum chamber, such that a workpiece can be held at up to 75 kV potential relative to the base and frame. The embodiment of the stage capable of accommodating a 300 mm wafer can weigh less than 100 lbs.
In a preferred embodiment, the stage comprises: a base; a frame attached to the base; a platform with a bottom platform surface; three substantially parallel adjustable limbs coupled to the base and the bottom platform surface, each limb comprising a raising member attached to the base, where the raising member is a constant force support in parallel with an electromagnetic raising actuator, a first flexural thrust joint with three degrees of freedom of movement attached to the raising member, a leg, a bottom end of the leg attached to the first flexural thrust joint, a second flexural thrust joint with two degrees of freedom of movement attached to a top end of the leg and the second flexural thrust joint attached to the bottom platform surface; and four electromagnetic platform actuators coupled to the platform and the frame, whereby coordinated operation of both raising actuators and platform actuators provides precise positioning and movement of the platform with six degrees of freedom.